The present invention relates to a spring steel having a good sag-resistance and a good hardenability.
There has been an increasing demand for light weight suspension springs reflecting a trend for light weight automobiles, in recent years. As an attempt to meet such a demand, it is said to be an effective approach to the reduction of weight to design the springs to have an increased stress and to use them under a high stress state.
However, if presently available spring steels are used under a high stress condition, there will arise problems such as deterioration of their durability and increase of sagging, and the increased sagging will result in decreased height of the springs and hence decreased height of the vehicle, with the consequent decreased height of the bumper causing a serious problem from the standpoint of safety.
Under the circumstances, there has recently been a demand for a spring steel having a high sag-resistance which makes high stress designing possible.
Heretofore, as a spring steel superior in sag-resistance, the steel corresponding to SAE 9260 (Japan Industrial Standard SUP 7) has become more popular along with the finding that silicon contained in spring steels is effective in improving sag-resistance.
Conventional spring, however, have a drawback such that when forming the steels into springs having complicated shapes or when forming the steels into heavy springs, much time is required from when heating is ended until when quenching is completed, thus allowing ferrite and/or bainite to be produced in the hardened structure, and therefore a desired hardness in not obtained.
However, there were severe requirements for light weight suspension springs. Accordingly, it has been strongly desired to develop a spring steel having a sag-resistance superior to that of SAE 9260.
With these circumstances as background, the inventors of the present invention have previously developed a spring steel superior in the sag-resistance to the steel of SAE 9260 and equivalent to the steel of SAE 9260 in the fatigue resistance and toughness required of spring steels, by adding one or more of vanadium, niobium and molybdenum in an appropriate amount to a spring steel of high silicon content, and filed an application thereon (U.S. patent application Ser. No. 06/289.852).
However, in the cases of a thick coil spring, a thick torsion bar and a thick laminated leaf spring, which are used for relatively large-sized automobiles or the like, it is difficult to harden the material to its core portion during the heat treatment and consequently the structure of the core portion tends to be bainite or ferrite-pearlite which has a lower hardness than a martensite structure, and thus the effect of improving the sag-resistance by adding vanadium, niobium or molybdenum is greatly impaired.